4 Answers
4

A different solution, which extends the cases environment. It adds an optional argument for defining array column options. The standard cases behavior is the default, so without optional argument it's like the normal amsmath's cases.

Just a note - I was trying this at codecogs.com/latex/eqneditor.php, and alignedat is apparently a "blocked word"; interestingly, if I replace \begin{alignedat}{2} with begin{aligned} (also for the \end), then it compiles in that editor, and the result looks almost the same as the image in this post. Not sure what the difference is, though... Cheers!
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sdaauFeb 20 at 19:21

I would recommend using a \phantom{-1 <{}} to achieve the proper spacing. This will reserve as much space as is taken up by -1 < (with the additional {} to get the proper spacing on the right hand side of the <):

If you also want the 1 aligned on the right hand side you can add \phantom{-} before the 1 to get:

\qquad doesn't leave enough space on the second line, hence doubling it to \kern4em. the {} after the less than sign in the \llap ensures the correct spacing between it and the following "x". finally, since \llap puts you into horizontal mode, $...$ are needed to restore math mode.